Move files from lib/haml_lint to haml_lint

Files in lib will be eager loaded and hence will require haml_lint to be
loaded. Since this is only a development dependency, we can't assume
this gem will be available in production, so it should never be loaded
in production.

.haml-lint.yml

@@ -7,7 +7,7 @@ exclude:
   - 'vendor/**/*'
   - 'spec/**/*'
 require:
-  - './lib/haml_lint/linter/no_plain_nodes.rb'
+  - './haml_lint/linter/no_plain_nodes.rb'
 
 linters:
   AltText:

lib/tasks/haml-lint.rake

 unless Rails.env.production?
   require 'haml_lint/rake_task'
-  require 'haml_lint/inline_javascript'
+  require Rails.root.join('haml_lint/inline_javascript')
 
   # Workaround for warnings from parser/current
   # Keep it even if it no longer emits any warnings,

spec/lib/haml_lint/linter/no_plain_nodes_spec.rb → spec/haml_lint/linter/no_plain_nodes_spec.rb

@@ -3,6 +3,7 @@
 require 'spec_helper'
 require 'haml_lint'
 require 'haml_lint/spec'
+require Rails.root.join('haml_lint/linter/no_plain_nodes')
 
 describe HamlLint::Linter::NoPlainNodes do
   include_context 'linter'